Detect platforms where memset(0) doesn't set doubles to 0.0.

This is allowed by the C statndard, which permits you to represent
doubles any way you like, but in practice we have some code that
assumes that memset() clears doubles in structs.  Noticed as part of
7802 review; see 8081 for more info.

changes/double-0-check

@@ -0,0 +1,8 @@
+  o Build improvements (bizarre platform detection):
+    - Try to detect it if we are ever building on a platform where
+      memset(...,0,...) does not set the value of a double to 0.0.  Such
+      platforms are permitted by the C standard, though in practice
+      they're pretty rare (since IEEE 754 is nigh-ubiquitous). We don't
+      currently support them, but it's better to detect them and fail
+      than to perform erroneously.
+